1. Field of the Invention
This invention relates to a magneto-optical recording medium which is used for the image recording, a large capacity data file and the like and a method of producing the same.
2. Description of the Prior Art
A magneto-optical recording medium which has various advantages, e.g., it can record information at a high density, has a large capacity, it can record and reproduce information without mechanical contact with a head, and it permits easy erasing and rerecording, has been put into wide use as a large capacity data file, a recording medium for image recording or the like. In various magneto-optical recording media, those which comprise a substrate and a recording layer in the form of a thin film containing rare earth metal and transition metal as the major components exhibit excellent magneto-optical recording properties. For example, a magneto-optical recording medium which has a recording layer in the form of a thin film of a rare earth metal-transition metal alloy such as TbFe alloy is disclosed in Japanese Patent Publication No. 57(1982)-20691. In Japanese Patent Publication No. 59(1984)-217247, a magneto-optical recording medium which has a recording layer consisting of alternate rare earth metal film and transition metal film (Tb and Fe) is disclosed.
However, the rare earth metal film and the transition metal film, especially the former, are very apt to oxidize, and the properties of the recording layer are apt to deteriorate with time. In order to overcome this problem, there have been made various propositions. For example, it has been proposed to prevent the recording layer from oxidizing by a pair of protective layers which are in the form of dielectric films formed of SiN, Al.sub.2 O.sub.3, SiO.sub.2, ZnS, AlN, AlSiN or combination of these materials, and are formed on opposite sides of the recording layer. See Japanese Unexamined Patent Publication Nos. 57(1982)-27945, 62(1987)-28753, 59(1984)-121368, and 60(1985)-80144, for example. Also it has been proposed to add Pt, Cr, Al or Ti to the rare earth metal-transition metal film as disclosed in "Jitsumu Hyoumen Gijutsu", page 228, Vol. 32 (No. 5), 1985, "Nippon Ouyou Jikigakkaishi", page 197, Vol. 11, (No. 2), 1987, and Japanese Unexamined Patent Publication Nos. 49(1974)-60643, 59(1984)-96714 and 62(1987)-293537.
However, the former proposition is disadvantageous in that aging permits oxygen and or water to permeate the protective layers and cause the recording layer to rust, and oxygen and water can reach the recording layer through a pin hole or crack in the protective layers and cause the recording layer to rust. Though the function of the protective layer can be enhanced when the thickness of the protective layer is increased, it can cause warpage of the magneto-optical recording medium.
When Pt, Cr, Al or Ti is added to the rare earth metal-transition metal film in accordance with the latter proposition, the resistance to corrosion of the recording layer is increased. However, if such metal is added to the recording layer by such an amount that the resistance to corrosion of the magneto-optical recording medium is sufficiently increased, the magneto-optical properties of the recording layer deteriorate and the Kerr rotational angle .theta.k is reduced, which deteriorates the C/N ratio of the magneto-optical recording medium.